text:
How to maximize your practical lift/weight ratio in a multirotor?
My friend and I are currently working together in a multirotor competition.  He's an experienced flyer and I'm more of a numbers guy (though my background is in software engineering, not aviation).
Our latest challenge involves maximizing our Lift to Weight ratio.  Essentially, we're going to measure how much our craft can lift and divide that by our craft's weight.
My preliminary research seems to show that the most obvious way of doing this is by adding more motors+props to the craft.  So, we're considering building a bare-bones dodeca-copter.
This seems like a very simple approach... maybe too simple.  I'm worried that although it's very intuitive it may also be very naive.
So, I ask the community, is our approach sound?  Are we missing some common optimizations that could help us?
In other words:  How to maximize your practical lift/weight ratio in a multirotor?
Update
We've been able to find more powerful motors (E800 instead of E310).  We have fewer of them, but I think we're still better off with them.  Here's the the breakdown of our build:

Main components (Flight controller, receiver, nuts, screws) = 313g
We're using E800 motors

Unit weight: 106g
Unit lift: 2100g

ESC weight = 44g
Prop weight = 14g
Battery weights = {410g, 584g}

The only frame material we have access to right now is square aluminium tubing.  Each "bar" of tubing weighs about 88g.
I did some calculations for a few different setups and this is what I got:
4 motors, 4 arms
This configuration requires 1.5 "bars" of frame materials.  This gives me the following total weight:
(4 * (motorWeight + EscWeight + PropWeight)) + (frameWeight) + batteryWeight + MainComponentsWeight
(4 * (106g + 44g + 14g)) + (1.5 * 88g) + 410g + 313g
= 1511g

The total lift is 4 * 2100g = 8400g which gives us a L/W ratio of
8400g / 1511g = 5.56

6 motors, 6 arms
This configuration requires 4 "bars" of frame materials.  This gives me the following total weight:
(6 * (motorWeight + EscWeight + PropWeight)) + (frameWeight) + batteryWeight + MainComponentsWeight
(6 * (106g + 44g + 14g)) + (4 * 88g) + 410g + 313g
= 2059g

The total lift is 6 * 2100g = 12600g which gives us a L/W ratio of
12600g / 2059g = 6.12

8 motors, 4 arms
This configuration requires 4 "bars" of frame materials but has two motors mounted per arm.  This gives me the following total weight:
(8 * (motorWeight + EscWeight + PropWeight)) + (frameWeight) + batteryWeight + MainComponentsWeight
(8 * (106g + 44g + 14g)) + (4 * 88g) + 584g + 313g
= 2561g

The total lift is 8 * 2100g = 16800g which gives us a L/W ratio of
16800g / 2561g = 6.56

Conclusion
If I'm doing my calculations correctly then it seems that our best option, given our limited resources, is an octocopter on 4 arms.
I'm not all that surprised since this configuration has the highest %age of total weight associated with motors.
Is 6.56 a decent Lift to Weight ratio?  Is there anything else we can do to improve it?
Competition Results
Weigh-in: 2460
Lift (meters of rope): 34m @ 346g/m
Lift (grams): 11764g
Lift/Weight ratio (excluding craft): 4.78
Lift/Weight ratio (including craft): 5.78
So, all in all, not too far off from our "ideal" prediction.

By adding motor+propeller+battery assemblies of the same L/W ratio, you indeed reduce the payload portion that each assembly has to lift, so you can lift more. The limit is when the individual lift starts reducing due to the turbulence created by neighboring propellers. Related: Does the ARCA hoverboard violate known limits on small-diameter electric fans?

Yeah, that's what I was worried about.  On paper our 8-motor 4-arm setup seems best, but the two motors per arm are insanely close to each other.  My teammate tells me there isn't much power loss associated with this, but I'm sceptical.  If we had more time I'd run practical tests with each setup.  Unfortunately, we only have the time for one build.

I'm not sure if this will help, just throwing out the idea that ducting the fans may help reduce the power loss by having the two props close together at the end of one arm. You will have the additional weight of the ducts, and will have to do some math/research to determine the best design for the ducts, but they may help. Not an aerospace engineer!

Adding motors, and associated propellers, structure, and batteries (assuming the same endurance is needed) will increase the weight almost as fast as the lift, because the components you are adding are almost all the weight. In fact, you'll need to make the structure stronger and that means double size structure will weigh more than twice as much, leading to lower L/W for larger design.

However, you have the "main components" that make rather large fraction of the total weight. If that's something you can't change and does not scale, you sure need a bigger craft.

Actually, that was a spreadsheet error.  The actual "main components" weight is 313g.  I'll update the question to reflect this tomorrow morning (it's late now).  Thanks for bringing my attention to that.

Beyond that, you should look at efficiency (try different propellers, especially different sizes) and try to save weight everywhere you can like using absolute minimum of batteries, weakest lightest structural elements that will hold it together etc.

@MetaFight, you'll notice the lower "main components" weight will make the L/W increase less significant, though it will still be there, because 313g is still rather large part of the total weigh. Then there is of course still issue of the batteries—you don't scale the batteries, which means the double-size design will have half the endurance and that means either the smaller has unnecessarily long endurance and you should calculate it with smaller battery, or the larger has insufficient endurance and you'll need larger battery.

@JanHudec yes, that makes sense.  In our case we only need to fly for about 45s so endurance is not an issue.  We've completed our build and will be putting it to the test later today.  I'll let you know how it went :)

To maximize lift, use the most powerful motors and let them turn the propellers at the highest possible speed. To minimize weight, make the structure as light as possible (use round carbon fiber/epoxy tubes instead of square aluminium ones, for example) and use only as much batteries as needed to achieve lift-off.
I would expect that there should be some more rules to ensure that the winning entry is actually useable. You did not mention any of this. Now efficiency comes into play, and the number of parameters to watch goes up. A lot.

Make sure that your propellers act on as much air volume as possible. Let them run at a reduced speed for better efficiency. You need to test your particular engine-propeller combination where the sweet point is - there is no general number for best efficiency.
Put as little drag in the way of the airstream as possible. This requires careful study of all shapes and cross sections. See on the right side of the picture below for a general overview of the drag coefficient of different 2D shapes.

Figure 33 from Sighard Hoerner's Fluid Dynamic Drag, Chapter 3.
As you can see, round is better than square. However, depending on the local Reynold number, it can help to add turbulators or local roughness to trip the boundary layer.
Also, adding motors to one arm will reduce the efficiency and thrust of the second propeller. Your 8 motors / 4 arm design will end in a disappointment - you cannot simply add the thrust of an individual motor-propeller combination once they interfere with another unit.
Without detailed knowledge of the rules of your competition, it is hard to give more detailed advice. I expect that a lot of educated trial and error will produce the best outcome.


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Is this explanation of adverse yaw correct?
It is explained here that adverse yaw is caused by the difference in induced drag due to the difference in lift being generated by each wing.
I've also seen a couple of websites (including Wikipedia) that argue that this explanation is wrong. They explain the cause of adverse yaw as being the change in the direction of the lift vector during a roll, or what some refer to as the "twisted lift" concept. They usually have a diagram like the following:

source
The twisted lift explanation discounts a difference in induced drag due to a change in the amount of lift. They contend that lift and drag remain the same but they twist in different directions. The descending wing has a slightly higher local AoA and the rising wing slightly lower. Since lift is perpendicular to the local airflow the lift vector rocks forward on one wing and aftward on the other.
This explanation seems flawed to me.

First of all, in the diagram they show the force on each wing being the same. If that were the case the aircraft wouldn't roll at all, it would ONLY yaw.
Second, the vector of lift being perpendicular to the airflow is correct, but that is really just an arbitrary division of the total aerodynamic force. The yaw axis of the plane is not dependent on airflow. It would be dependent on the portion of the force vector parallel to the wing chord, aka the axial force. As shown in another question, depending on a number of factors the force vector in relationship to the wing does not necessarily move forward with an increase in AoA.
I can't come up with solid numbers, but trying to figure this out for normal flight speeds it would require a very fast roll to even make a 1° difference in local AoA. I ran numbers for a 3° per sec roll at 120 KTAS on a 36ft wingspan (C172) and came up with a change in AoA of about .27°, and that Is only at the wingtip. It would decrease the closer it got to the fuselage. It doesn't seem like it would be enough to cause a significant change in vector and a resulting yaw moment. Or at least it would be secondary in magnitude to the difference in the drag due to reduction of lift.

Since the changes in lift are being created either by changing the effective camber of the wings (ailerons) or by altering the trailing edge flow (spoilerons) rather than a change in AoA I don't think it's quite so simple to predict that the direction of the aerodynamic vectors will necessarily move in opposite directions as this theory assumes.
If this explanation is correct can someone show me what I'm missing?

Both explanations in the OP are correct to the overall phenomenon of adverse yaw.
There are three sources of adverse yaw:

Difference in induced-drag due to ailerons: down wing aileron reduces lift while the up wing aileron increases lift, which generates a difference in induced drag in each wing. This yaw moment counters the desired yaw motion. This is manifested through the control derivative $C_{n_{\delta a}}$.
Yaw-roll damping: as roll rate builds up to steady-state, the down wing experiences a larger flow incidence while the up wing experiences a smaller flow incidence due to the rolling motion. This is the twisted-lift concept mentioned in the OP: because of the difference in the local AOA, the lift and drag vectors are twisted. As Jan Hudec correctly pointed out in his answer, at steady-state roll, the total lift on each wing is equal, so the yaw moment really comes from the twist. This is manifested through the stability derivative $C_{n_p}$.
Converted sideslip: if there is a non-zero AOA, the rolling motion will convert some of the AOA into sideslip; this sideslip is opposite to the direction of the turn. This effect is especially pronounced when the roll rate is large. Some high performance FBW systems will address this by implementing stability-axis roll instead of body-axis roll, especially on fighter jets whose roll rates are high.

First of all, in the diagram they show the force on each wing being the same.

In a steady roll, i.e. when the roll rate is constant, the forces are the same. A non-zero moment causes angular acceleration, so when the aircraft is rolling with constant rate of roll, the moment must be zero, which means the lift on both wings have to be equal.
That rules out difference in magnitude of lift as the source of the difference and leaves the difference in angle as the only plausible explanation. The explanation is correct.

When you first deflect ailerons, the wing with aileron down does indeed start to produce more lift and the wing with aileron up less lift. This difference causes rolling moment, which causes the roll rate to increase. It already starts causing adverse yaw due to the difference in magnitude of lift too.
However as the roll rate builds up, the angle of attack increases lift on the wing going down and decreases it on the wing going up until the lift equalizes and the roll rate stabilizes. At that point the lift is the same and the adverse yaw is caused by different direction of lift due to different angle of attack.
The fact that ailerons are deflected throughout the roll might lead you to think that there is still difference in lift. But there isn't. The aileron deflection just changes the roll rate in which the forces are in equilibrium.
Also while the angle between realtive wing and the total aerodynamic force (which corresponds to the relation between lift and induced drag) is in general not constant, in this case all the factors it depends on — wing span, air speed and density — are (almost) the same for both wings, so the difference in angle of attack is the main factor for the direction of the resulting force.
And last, but not least: How It Flies is very reliable regarding the physics (and most other things too). I would be very surprised to find an incorrect explanation there.

Ah, I missed the difference between an increasing roll rate and a steady roll rate. That dang inertia thing gets me every time. Does the adverse yaw remain linear throughout the roll or does it change as the roll rate steadies?

@TomMcW, I don't see good reason why it should be constant, but calculating how it changes would be rather complicated.

Maybe you could add that there are two contributors to adverse yaw: Initially it is $c_{n\xi}$ until the roll rate picks up, and then it is $c_{np}$. In both cases the lift vectors tilt forwards and backwards, and both explanations in Tom's question are equivalent.

Is my thinking correct when I say that yaw moment is the axial component when AoA is non-zero?

@TomMcW, I am not sure axial component of what, and around which axis, you mean, but most likely no. Moment of force appears when the sum of forces acting on the object acts in line not going through the centre of gravity. A yaw moment is a moment that tends to turn the aircraft around the vertical axis and is caused by difference in drag between the left and right side (wing), which offsets the action point of the total drag to the side with higher drag.

@JanHudec I'm talking about the axial component of the aerodynamic force vector. If you break the vector down from the pov of the chord you get normal and axial components, as shown in the second diagram in my other questio. It seems that the axial component, which is only a portion of the drag, is the part that is working in the yaw axis, since the wing chord is fixed in relation to COG

@TomMcW, a moment around some axis is caused by force perpendicular to that axis (along line offset from CoG). Chord is not parallel to the vertical axis. The horizontal axis is, so we are talking about the force along that (yes, it is not drag, because drag is the component parallel to the oncoming flow while we are concerned with the aircraft coordinates).

The roll is steady state because of the aero damping.

The lift would still not be balanced between the wings during a steady-state roll, because, as @Koyovis said, there is aero damping. Is that difference in lift is enough to account for adverse yaw? Probably not.

@markasoftware my understanding of aero damping is the equalization of lift due to variation of angle of attack—though it's true that since the angle of attack now varies along span, it affects the relation between lift and induced drag, so there may be difference in drag even though the difference in force goes away (and it's difficult to say generally what the change is).

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Which airplane is this lego model?
I'm wondering about the type of jet in this lego kit:
https://shop.lego.com/en-US/Airport-Air-Show-60103

One of the jets appears to be an F-18 (the one with twin tails, of course) but I can't place the other one. It looks vaguely like a Eurofighter, except this model only has a single engine.
I realize that these are stylized models, but I was hoping someone could at least identify what they are "based" on.
Thanks!

I wouldn't assume that they're necessarily based on a single real-world aircraft. They might be, but they may also be a composite of two or more types that the Lego designer thought would make a good model.

Single engine delta wing canard might be the JA-37 (Viggen) or JAS-39 (Gripen). That said, I don't think they're actually trying to resemble any one model.

One of the reasons I asked this question is I'm not as familiar with European Aircraft. Since Lego is based in Denmark, a lot of their models seem to be inspired by European designs.

I'm unable to find a biplane which comes with a 3-blade propeller with a vertical stabilizer that is not rounded.

yeah that biplane is weird. It has a canopy so its more like a modern stunt plane, but it didn't look like anything I recognized. I also haven't gotten to that part of the build yet so my curiosity about it is not yet roused.

@falstro hmm, I would accept an answer for the Grippen (the viggen is just fugly and is offensive to me, but besides that the canards appear to be slightly higher on the craft than the wings which is not the case for the model).
Yep, The Grippen looks like a fairly good match.

To be perfectly honest, LEGO does have a habit of mashing similar airplane designs into their own models. The twin-tail airplane has most of the features of the F/A-18 with its trapezoidal wings and slanted, trapezoidal vertical stabilisers. The F-14's contribution to that design is mainly in the engine intakes.
The single-tail airplane has most of its design from the Saab JAS 39 Gripen but the vertical stabiliser looks like it comes from the Dassaut Rafale (Knowing LEGO pieces, it could also be that they couldn't find a proper piece to fit).
Of course LEGO also has this unholy mash-up of the F-22 and F-35. So it does not surprise me that they have multiple plane designs in these airplanes.

The twin tail looks more like a F-14 to me:

Note the sweep angle of the wing and the rectangular engine inlets beneath it. The F-18 has rounded engine inlets, and the front portion of the wing extends all the way to the cockpit:

The single engine looks like a Saab JAS 39 Gripen:

Note that the front smaller wing of the Typhoon is in front of the cockpit:

"The single engine plane is a F-16" You mean the one that's clearly a delta wing canard...? ;)

I thought the single-tailed plane might be an f-16 until I actually started building the kit and I noticed there are two fairly large canards on the front of it, as well as the wings being much more delta-shaped.
As for the twin-tailed aircraft, I hadn't thought about an F-14, but the wings are very obviously meant to be fixed-wing.

Note that the single-vertical-tail model has a design resembling a delta wing with canards. Not an F-16. Try a Saab Gripen perhaps?

@falstro thanks! didn't notice that!

@KevinMilner the wings aren't movable in the lego, but I think it's more of a limitation of what they can represent. The large air intake + the rectangular shape speaks F-14 to me (F-18 is relatively more curved).

The newer Super Hornet has rectangular inlets and the outward angle of the tails is much closer to the F-18.

ah! @fooot beat me too it I was also going to point out that the super hornet has rectangular intakes

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Why do aircraft take this strange route into JFK?
Looking at FlightRadar24, I see there is a long line of aircraft heading into JFK, but that they seem to all be starting to head straight at it from over Pennsylvania (as can be seen in the image below, and at https://flightaware.com/live/flight/JBU1042/history/20170621/2301Z/KSAV/KJFK.
It's not just JBU1042, though (which looks like it's circling too?). A bunch of aircraft seem to be doing this. And not just ones coming from the western U.S. Many are coming from the south too.
It doesn't appear that way when I plug the route that FlightAware displays into something like SkyVector.

What in particular seems strange about this route? What were you expecting to see? We're you expecting a more direct routing?

@JonathanWalters just that it doesn't seem to follow any arrival procedures I could find, and that it seems a little weird to me that they are being sent over mid-Pennsylvania before being turned and heading straight for the airport. I guess I've just never seen them that far out before, but mainly that it doesn't seem to match up with the routing on FlightAware as well as SkyVector. It also seems interesting that despite the route shown in FA, the blue dotted line does seem to indicate that's where they planned to go, yet it seemingly isn't in the route displayed by FA.

Well, without comparing it to the charted procedure, that flight path in your first picture looks to me at first glance to be the Lendy Six arrival into JFK. You might look that up and compare the charted arrival route. I would guess that's what they are flying. It starts over the Wilkes-Barre VOR.

Yep defiantly looks like LENDY6

I feel that a lot of these questions would be obvious if FlightRadar and alike would overlay NEXRAD on the maps...

@RonBeyer what do you mean by "displaying NEXRAD on the maps"?

@lightbord Weather Radar

@RonBeyer FlightAware has some sort of WX radar on it. Not sure of its data source. I agree it would be nice to have it on FR24 as well.

@RonBeyer Is that not that what we are seeing in the first image, from FlightAware, NEXRAD or similar?

Possibly it has something to do with that line of storms blocking the more direct routes into JFK from the south?

Stormy change of arrival plans
Originally, the flight was planned to use the CAMRN4 STAR coming in from SIE VOR south-southwest of KJFK.  However, there is a strong band of storms just south of that VOR on the FlightAware map, blocking that route.  So, the flight got a reroute to pick up the LENDY6 STAR instead, getting vectored northwesterly to around FQM (Williamsport) VOR first and then back onto the course to pick up the STAR in order to provide sufficient space to descend to FL230 by JENNO.

Thanks for the detailed answer! I'm really trying to learn how to understand reroutings and stuff, so may I ask how you were able to determine this? I get that you can tell its LENDY6, but how did you know it was FQM that they got redirected to? And as near as I can tell it looks like they exited the J121 at WEAVR and then direct FQM. But they did do that little loop thing down over North Carolina. No idea what that was. Would my analysis be anywhere near correct or are there other things happening I may not know about?

I actually just found a thing on flight aware to show enroute charts. My analysis wasn't correct. Except for the little half loop thing in North Carolina, I think I can pretty much figure out their routing. They exited the J121 a little past BARTL. From there they did that loop type thing, possibly direct DIW. Over DIW, they headed straight for J109. Once over and lined up with J109 (008 from ILM), they went direct VALLO, joining J220 for a second. At OGESY, they split onto J227, then at BABEE, they tuned and headed straight for LVZ, where they followed LENDY6. Only unsure about thing over NC.

@lightbord The NC loop may have simply been a holding pattern further out to allow easier spacing since flights were being rerouted due to weather.


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How many private jets land at Heathrow airport per day?
Heathrow airport is one of the busiest airports in the world, one would assume most traffic there is reserved for commercial airliners because they carry a much larger number of passengers.
How many private jets land at Heathrow airport per day?

On a normal work day
On weekends.

"one would assume most traffic there is reserved for commercial airliners" Citation needed for the implicit claim that airports optimize for a high number of passengers.

I don't  know, but it isn't many. In all the times I've flown to and from there (as a passenger) I think I've seen 1. That's because it's an expensive airport to fly into and as you say it is busy, plus there are many very good alternatives like Biggin Hill, Farnborough, Wickam Air Park, London City, etc.

How many private jets land at Heathrow airport per day?

None!

January 2018 movements, source: CAA

Related: Where does an A-lister land when they waft in from paradise? Yes, Luton

To try to explain why this absence of business planes at a large airport, its important to understand that private aviation in Americas and in Europe are not seen the same way.
In Americas private planes are very common, this is not the case in Europe, where you must be lucky to know someone in your neighboring who is a pilot. As a consequence airliners and other planes have separate lives with different fees paid.
For example, I live in Paris: Business planes use Le Bourget aerodrome (the one which hosts the Paris Air Show -- btw nobody here knows what is the Paris Air Show, as everyone calls it Le Salon du Bourget since it exists... --, while airliners use CDG or Orly airports (and now the remote/low cost Beauvais). This is the same in UK.

It's not that different in the US. Major airports here don't exactly welcome GA flights either, especially when there are smaller airports all around them that are more convenient, less congested and less expensive. They just can't completely ban GA like in Europe because that's against the law here.

@StephenS: It's not forbidden for a private-use aircraft with appropriate equipment to land at CDG or Orly either, but it's expensive and the time waiting before take off is ridiculous (not mentioning the border control time). For landing, there is a condition: The airspeed must be maintained above 200 kt until 4 NM of the threshold, which means for a small aircraft, an additional workload at the last minute to switch from the clean configuration to the landing configuration.

Thank you for the answer


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What is a "Russian IATA code"?
On Wikipedia, a few obscure ex-Soviet airports like Tamchy Airport in Issyk-Kul, Kyrgyzstan list "Russian IATA codes":

IATA: none (ИКУ), ICAO: UCFL

"ИКУ" being Cyrillic for "IKU", which is not a recognized code according to the official IATA finder.  This Russian site lists "city codes" (Коды городов), but gives no source, and as bonus gives the ICAO code for Иссык-Куль as "UAFR" (which gcmap.com has never heard of).
Where do these codes come from?  And does IATA play any part in maintaining them?

interesting question. My guess is it's maintained by the Russians themselves much like the US has many fields with no international code because there's no international traffic.

It's a Russian internal city or airport identifier. Many small cities don't have IATA codes, but do have internal Russian codes.
This system is called CRT (Center for schedules and tariffs, ЦРТ, Центр Расписаний и Тарифов) named after a department in the organization that manages them (see below). Wikipedia article cited below claims it's also called "Sirena" code (named after Sirena -- a booking system for airline and railway tickets).
Here's a translation of a fragment from Russian Wikipedia:

The internal city and airport coding system in the former USSR is structured analogous to IATA. [...]
The airport code in this system consists of 3 letters of the Russian alphabet. As in IATA codes, the 3-letter combination is interpreted as atomic and has no internal structure. As in case with IATA, all 3-letter codes are unique.
Like IATA, the codes were assigned to cities such that the code has something in common either with the name of the city or with its IATA code [...] Contrary to IATA, the internal codes were changed after cities were renamed in the 1990s: Saint Petersburg's internal code was changed from ЛЕД to СПТ, Samara (formerly Kuibyshev) -- from КУФ to СМШ, Ekaterinburg (formerly Sverdlovsk) -- from СВХ to ЕКБ.
Many small cities and airports in the former USSR have an internal code only.
In late 1990s foreign (non-Russian) cities were assigned internal codes because of the popularity of these destinations (e. g. Barcelona -- БЦН).
As of April 2009, there are about three thousand internal codes that are actively used.

It appears that the codes are maintained by the Transport Clearing House. The section Coding Directories on their page Schedule & Tariffs  says:

STC maintains the aggregated database of coding directories for the purpose of universal use of codes in information systems for passenger services reservation and sales, publication of timetables and fares, ticketing, settlement and other commercial issues related to passenger services.
STC maintains the codifiers in pursuance of Provision on Assignment, Maintenance and Use of Internal Codes of Carriers, Organizations, Airports, Cities (points) and Civil Aviation Agencies (№ ДВ-63/и dated 12.05.94) and Directive "On Coding Directories Approval” (№ ДВ-109/и date 14.07.94).
STC supports the following coding directories:
Airlines
  Airports
  Cities

In Russian, STC is translated as CRT (ЦРТ) and I've heard people refer to the internal codes as "CRT codes".

+1, but would be worth mentioning that Центр Расписаний и Тарифов = "Center for Timetables and Tariffs" (Fares).

@jpatokal Thanks for suggestion. I've edited the answer.

Long story short, there is no such thing and @jwenting's comment is almost certainly correct (see edit below).
The only Google hits for "Russian IATA code" (apart from this question) are a series of Wikipedia articles on Russian airports, either on wikipedia.org itself or on other sites that have obviously scraped Wikipedia for content. All the articles (well, the ones I checked anyway) were edited by the same user and they refer to the same page as a source.
The table on that page has a column called "IATA code" which is actually empty for Tamchy. The code you mentioned is in another column called "internal code", presumably that means it's a domestic code used only in Russia and comparable - as @jwenting said - to the many 3-character US codes that have no IATA equivalent.
In other words, the Wikipedia contributor invented something that doesn't really exist; even the quoted source doesn't refer to ИКУ as an "IATA code".
EDIT: The Russian Wikipedia page on IATA codes goes into slightly more detail on the internal codes (translation from Google):

Sometimes this system of codes called "sirenovskoy" (from the ticket
  reservation system "Siren"), but the name is historically incorrect -
  this coding system used before the appearance of "Sirens"

The article also confirms @jwenting's comment:

Many small towns and airports in the former Soviet Union did not have
  the IATA code and have only internal code.

However, that page doesn't say who manages the codes and it doesn't provide any 'official' sources either, just links to other pages that simply list the codes. A cursory search of the Russian ministry of transport website doesn't turn up anything, but someone who actually speaks Russian might have more luck than me :-)

Sure, I wasn't expecting there to be a literal "Russian IATA code", but is there some organization that decides them?  Or is it just one random guy on the Internet making things up?

@jpatokal probably the organization that oversees aviation in Russia.  In the US the FAA assigns all airport codes, and IATA codes may be different or not exist at all for a given airport (e.g. FAA: UNV, ICAO: KUNV, IATA: SCE, or FAA: 50R, ICAO: none, IATA: none).  Remember the IATA names are not regulatory names, they are industry names.

@casey Dig up that organization's website and their official list of codes, and the bounty is yours :)

I'm a Russian speaker - I've spent half an hour searching and found no official list of internal airport codes. I can try to ask on Russian aviation forums if you're really interested.


text:
Why does the Pilatus PC-12 go faster than the King Air C90?

Pilatus PC-12 (source: www.what2fly.com)

King Air C90 (source: www.ok-aircraft.cz)
I am wondering this because of the blades on the propellers on both of the planes. The Pilatus PC-12 can travel up to 360mph with its 4 blades on the propeller. The King Air C90 has 2 engines with 4 blades each, is around the same size and can travel at a slower speed of just 300mph. Why is this when the King Air has 2 engines instead of 1 with 4 blades and travels slower, and the Pilatus only needs one engine with 4 blades to go 360mph?

How much HP do the engines produce? how heavy are the airplanes? Are the propeller blades airfoils different? Are the propeller lengths different? How much drag does each airframe produce? How are the wings different?

Where are you getting these speeds? Everything I can find says they both have max cruise speeds of about 270 knots.

@fooot: Wikipedia - via Beechcraft - claims a cruise of 226 knots and a max (presumably Vne) of 270 for the King Air C90GTi.

Land miles or nautical miles?

The speed of an aircraft is not only dependent on the number of propellers. It is a function of (among other things),

Excess power available- The Pilatus PC-12 has more powerful engines compared to the King Air C90. The PW PT6 engine used in the Pilatus produces 895 kW, while (a different variant of) the same engine used in King Air produces 410 kW each. The power available for the PC-12 is more.
Propeller efficiency- The PC-12 uses a different propeller compared to the C90, which may have more efficiency at high speeds. Also, the PC-12 propeller diameter is more compared to the C90.
Aircraft design- The PC-12 flew around 30 years after the C90. The aircraft design has improved in many ways during the intervening years in aerodynamics, materials etc. These improvements can be used to increase the cruise speed.
Power to weight ratio- The PC-12 has a better power-to-weight ratio compared to the C90, which translates into better speed.

It is difficult to pinpoint any single reason for increased speed. However, the increased (cruise) speed in PC-12 can be explained using a combination of the above reasons.

So the propeller rotates the blades faster on the PC-12.

@Ethan I doubt it. They use the (variants) same engines for one thing. Usually more thrust is obtained from propellers improving the (airfoil) shape rather than increasing speed as it will cause a number of problems (like high tip speeds). Most of the propellers in use today have constant speed and variable pitch

Also don't forget the engine pods of King Air add a lot of drag compared to the nose-mounted engine of PC-12.

It's not the number of engines, but the power of them. A Boeing 777 has two engines, but can fly faster than a B707 with four engines. Similar to a four wheel drive vehicle not necessarily being faster than a 2-wheel drive - the contrary actually, since there are more drive train losses. More engines in an aeroplane also means more losses: engine nacelles, higher vertical tail to enable flying with one engine etc.
How the power of the engine is transferred to propulsive thrust is a matter of aerodynamics, which also puts a limit on the maximum speed of propellor planes. If the propellor tip approaches or breaks the speed of sound, the shock wave resistance consumes a lot of power which is then not available for generating thrust. No matter how many engines the propellor aeroplane has, it will always run into this speed limitation. Which can be improved by propellor technology, not number of engines, as visible on this photo of the Airbus A-400.

"Similar to a four wheel drive vehicle not necessarily being faster than a 2-wheel drive - the contrary actually, since there are more drive train losses." I think that the IMSA GTO Audi 90 Quattro would prove otherwise.

Does it prove that there are fewer losses in a 4-wheel drive train? The argument is not that there are no 4WD cars faster than 2WD cars.


text:
Why would only a specific model of drone be allowed to fly over crowds?
The FAA allowed CNN to fly a drone over crowds at extremely low altitudes. Its scope is within the UAVs. Only a specific model/type is allowed to operate on these flights.
Why is this model of drone allowed to do such a thing, and others not? CNN claims that it is to minimize the damage to people, if the drone crashes or something.

The FAA would regulate the kind of underwear you have if they could come up with a plausible rationale for doing so.

@TylerDurden: No, that's not true. I found FAA people quite reasonable, especially when compared with other bureaucracies. They have a mandate and act accordingly.

Oh, Peter, I disagree.  The absurdity that can erupt from the doors of a FSDO is stunning.

Probably they asked for permission to fly a specific kind of drone and that's what they got.

CNN has applied for and received multiple waivers for 14 CFR § 107.39 Operation over human beings as allowed by § 107.205 List of regulations subject to waiver.

Issued August 29, 2016 — This allows "Small unmanned aircraft system (sUAS) operations over human beings in accordance with the CNN UAS Operations Manual, Fotokite Pro 1 Operator Guide, and Fotokite Pro 1 Specifications Sheet." It allows operation "over private or controlled-access property with permission from a person with legal authority to grant access" and prohibits operations over open-air assemblies of people.
Issued July 20, 2017 — This allows operations "conducted over human beings for the purpose of closed-set motion picture and television filming". Make and model of sUAS not specified, but all operations must comply with the operator's manual.
Issued October 13, 2017 — This waiver is granted for the Vantage Robotics Snap only, and allows "operations over all human beings (including Nonparticipants)". It also limits the altitude when operated over people to 150 feet AGL, as opposed to the normal limit of 400 feet AGL. I assume this is the waiver you are interested in.

The simple reason why only one particular model of UAS is allowed is that CNN only submitted that model for waiver and no others, and the FAA approved that.
This is a somewhat unsatisfying answer, but neither CNN's nor the FAA's reasoning is included in the text of the waiver itself. The rest of this answer is speculation on my part.
As can be seen on Vantage's specifications page for the Snap, the Snap is reasonably lightweight (1.37 lbs / 620 g including battery), has caged props, and has a high-quality camera. (The camera isn't a matter of safety, but it is presumably something a cable news provider is interested in.)
If a prop were to fail catastrophically, the cage would hopefully contain at least some of it, thus eliminating risk of a crowd being hit by sharp objects. The height restriction of 150 feet AGL reduces the potential for injury of a nonparticipant should the sUAS fall on them, and the prop cages prevent contact with moving parts.
I suspect that CNN applied for a waiver specifically for this model and the FAA approved it based on the following:

It's specific. The more general the application, the more likely it is to be rejected. If you know what you're working with, you can evaluate all the parameters and come up with a safe solution. If there are more variables, it's harder to reason about.
It has certain safety features as discussed above, which make it more suitable for flight over nonparticipants than some other models.
It's standard. CNN has a single platform to train their over-crowd operators on. Once again, this also has safety benefits, because the operators are extremely familiar with one model of unit, instead of broadly familiar with many different ones.
This particular model of sUAS had all the features that CNN wanted and that the FAA was interested in for this application. This may include such factors as weight, safety features, well-written operation and maintenance manual, and ease of maintenance. Additionally CNN may have liked the cost (the FAA wouldn't care about that).

The FAA maintains a list of all Part 107 waivers granted.

Special permissions can be granted for platforms with sufficient redundancy.
For example, if you have 4 props and one fails you'll crash. But if you have eight (in the correct pattern) and one fails, you can abort the mission and land safely with minimal increased risk to those on the ground.
Same applies to batteries, autopilots, GNSS receivers, etc - if you have no single-point failures, it becomes safer to operate in risky locations (in your example, over people.)

That logical, but what is the related regulation supporting this claim?

Agreed -- it is more likely that these are contributing factors that led to a allowed deviation from 14 CFR 107.

I wonder if prop failure is a common thing (drone noob here).

@Kyslik Engine failures happen even for adult aicraft with people on board. And remember https://www.youtube.com/watch?v=p9T6-KPFRq8

@Kyslik I'd imagine it's significantly more common than the failure of massive turbine engines that cost tens of millions of dollars and yet lots of regulations exists for the scenarios when those fail (and fail they do.)

And does the engine fail or propeller itself. Is "prop" term used for both at the same time?

There is nothing particularly special or redundant about the Vantage Robotics Snap, which appears to be the model under discussion.

"if you have 4 props and one fails you'll crash" - not necessarily

The most likely explanation is that CNN has a waiver from the FAA for certain operations. A waiver is a limited permission to deviate from the regulations and they're very common. In fact, for some operations they're required.
CNN is most likely operating drones under part 107. Within that part, 107.205 lists the regulations that can be waived and flight over crowds is one of them:

§107.205   List of regulations subject to waiver.
A certificate of waiver issued pursuant to §107.200 may authorize a
  deviation from the following regulations of this part:
  [...]
  (g) Section 107.39—Operation over people.

(By the way, 107.39 prohibits flight over people at any altitude.)
I suspect that CNN asked the FAA to grant a waiver to 107.39 to allow them to fly over the crowd, and the FAA allowed it but only if CNN uses a drone that has specific engine redundancy or other safety features. The FAA can include whatever conditions they like in a waiver.

CNN requested a waiver for operations using a particular drone type. The FAA approved it with conditions. The FAA did not dictate the drone type. (I suspect the FAA would get in trouble with manufacturers if they started recommending particular drone types.)

@Dranon I reworded it. I intended "type" to be taken loosely, rather than being a specific model. Nice find on the waivers, by the way!

authors: Vladimir F Героям слава


text:
How often, and how promptly, do airports have to renumber runways because of magnetic pole drift?
This question was prompted by this answer to another question by Jamiec.
As the magnetic poles drift (usually pretty unpredictably), runway numbers become inaccurate. As noted in the linked answer, this has resulted at the very least in the renaming of the 15/33 runways at Madrid to 14/32.
How often does this happen, and at what point is it no longer reasonable for pilots to make the corrections necessary to make a smooth landing, requiring a renaming of the runways?

Runway renumbering. I've seen charts showing magnetic declination variation in the past, and it's certainly constant!

My experience (in the Florida area) is that magvar shifts enough to require runway number changes every 10 years or so -- it is location dependent.  The local magvar has shifted about 2.5 deg over the last 25 years.

@Jules: That's not constant

My previous comment should say every 10 years at one of the local airports.  Since the Runway numbers are to the nearest 10 degrees (+/- 5 deg), it depends on the actual runway alignment relative to the transition point.  For some airports, it may not happen for another 80 or 90 years.

@mins I think the original comment was deleted -- "constant" referred to the variation in declination, not the state of the declination

In the style of the duplicate; short answer: when its appropriate, medium answer: when it has drifted >3 degrees, long answer: read Lnafziger great answer :)


text:
Why does an engine fail if its FADEC fails?
Due to scarce resources on Full Authority Digital Engine Control (FADEC) and different FADEC technologies used in different engines, I have a poor understanding of it.

As the title says, why can't we allow an engine to run by other means if its FADEC fails?

If the FADEC in one engine fails, does that mean the other engine also has a chance to fail?

Are FADEC and engine monitoring unit the same?

I am seriously lacking the knowledge about FADEC. So, a detailed answer with its working is highly appreciated.

FADEC role includes all actions previously done by the flight engineer, and more. FADEC at FAA, and at EASA. A 1978 article on the coming Pratt & Whitney FADEC (click the pdf button below the description). Likely more documented than Wikipedia.

Bullet 1: Because FADEC eliminates any mechanical connection between thrust lever and fuel control system.  Part of the attraction is elimination of many yards of control cable or teleflex (push-pull cable) runs.  The other attraction is the computer does a better job.
Multi-channel computers are running the engine. The pilot input when the thrust levers are moved is just a command or request to the computers.  As with flight control fly-by-wire, safety and redundancy is achieved with multiple redundant computer channels, sufficient to bring the safely level to equivalent to having a manual backup system.
Bullet 2: No.  The systems are completely separate.
Bullet 3: No. An engine monitoring system just monitors and displays or stores information, with no active control.  FADEC is complete control of the engine by computers.
Historically, there are basically 4 main engine control evolutions:
Primitive (1940s) - The first jet engines had not much more than a lever that operated a valve on the fuel controller and the pilot's eye-brain-hand interface managed the engine parameters.  If you weren't very careful with inputs, you could overtemp or flame out an engine easily.
Modern Hydromechanical (1950s) - You move the trust lever and it moves a lever on the fuel control unit and and the resulting fuel flow is the result of the input, but with various internal hydraulic/pneumatically sensed adjustments to regulate the engine's response. These systems allow you to shove the thrust lever around aggressively without so much worry about flaming out or melting the turbine.
Electronic Engine Control (1970s) - You move the thrust lever, and it moves a lever on a hydromechanical fuel control unit, which commands a "baseline" fuel schedule.  This schedule is added to or supplemented by computerized trimming input to get the final fuel schedule.  A computer controlled the last, say 5%, of the engine's fuel flow using a torque motor or some other electric servo device on the fuel control unit.  The computer was able to fine tune the fuel flow, providing a bit more efficiency and faster response compared to a manual fuel schedule.  If you disable an EEC equipped engine's computer, the power will roll back a bit to the untrimmed manual fuel schedule. So if N1 is 88%, and you switch off the EEC or it fails, the N1 might roll back to 84% and you'd have to move the thrust lever up a bit to regain the the original thrust setting, and the engine would be a bit more sluggish in its response.
Full Authority Digital Engine Control (1990s) - No mechanical connection at all. Same as FBW flight controls. You move the trust lever and it's just sending a voltage to a computer, telling the computer you'd like some juice, and the computer decides what it's going to do with fuel flow to give you the juice you want.  For safety you will have 2 or more independent computer channels cross checking each other with one in charge and the other monitoring and ready to take over; whatever kind of back-up architecture is required to meet the minimum safety and reliability level.

EEC is still used as the main component in FADEC. These EEC and 1970s EEC are different terms?

typo: "trust lever" should presumably be "thrust lever" (unless you're hinting at your view of the system!)

@ChrisH "Emergency! Engage maximum trust!

@Auberron sorry that was all just dashed off the top of my head (I was in the aerospace industry for 40 years).  You'll have to hunt reference information yourself.  Pre FADEC EEC was just a trimming function with very limited authority, and may include both digital (software) and analog components within its architecture. And you still had a hydromechanical fuel controller with all of its internal mechanical controls and sensors.  FADEC does away with all that and does everything with software (the Digital part of FADEC), like FBW. And like FBW, safety is achieved with redundant architecture.


text:
What is the type of the seaplane at the end of runway 18L on Schiphol Airport?
From 47 to 52 seconds into this video we can see a seaplane parked in a small pool of water. What type of plane is this and when did the plane fly for the last time?

To clarify, it's a Fokker F-27 Friendship 500.
Manufacturered in 1971, PH-NIV has photos of it on Wiki:

Registration details here
As to how it's in Amsterdam Schipol:

towed to entrance at Oude Meer nr Schiphol East 12.07.12 as PH-NIV in
  old Fokker proto cs owner Coöperatieve Vereniging Anthony Fokker
  Logistics UA moved to permanent display location 13.03.13 on poles in
  a lake with nose up,

So it's been there since March 13, 2013 (the same day the current pope was inaugurated, incidentally).
Before that, it was registered with several companies during its service:

The aircraft is not actually a seaplane but a Fokker F-27 (PH-NIV) which is mounted on posts in the middle of a small lagoon.
It was built in 1971 and delivered to Air France. I couldn't find a last flight, but it was registered to Fokker in 2012, although there's no indication whether it was flying at that point.
For copyright reasons I haven't included images here, but photos can be seen at JetPhotos


text:
What is the go-around pattern and procedure for Heathrow?
An hypothetical situation in which an airplane taking-off at Heathrow must go-around / land immediately after taking-off. What will happen during this scenario:
Is there any specific action to be done in this scenario, i.e. setting transponder to some code?
What is the go-around / return pattern for Heathrow? Over which villages will the plane fly and at which altitude?

A "go-around" is when an aircraft approaching to land aborts the landing. That does not seem to be what you are talking about.

There are no standard routes or procedures for aircraft that need to return shortly after departure. What happens will be up to air traffic control and ultimately the pilot.

I think you have just answered my question. Will you care to turn comments into an answer?

Needing to return shortly after taking off is not very common, and as such there is no specific established procedure or route to be followed. The pilot will inform air traffic control of their intentions, and ATC will route the flight back for an approach. Depending on the situation, the flight might require priority, in which case other approaching flights will get bumped back in the sequence.
Transponder codes are used to identify flights on ATC radar. Simply speaking, one specific flight will have one specific transponder code during the whole flight - it is not changed because the flights needs to do something unexpected. Transponders are not used to transmit intentions (although there are a few special codes that can be used in case normal means of communication are unavailable).
For a flight needing to return to Heathrow, the return path will be similar to that of any other aircraft approaching the airport.

A return for landing after takeoff could be due to an emergency situation, or an operational need.  Part of the clearance that the flight receives is the initial routing after takeoff.  The aircraft will be expected to follow that routing.  If a problem occurs which does not permit climbing on that routing, the flight will also have a procedure, not known to air traffic control and usually not a charted procedure that's published, which the crew will follow.  This is the engine-out procedure, or what used to be called the "pink page" procedure.
The pink page, or engine-out procedure is something published for a specific operator, not availble elsewhere, and is what the operator will do in the event of an emergency while departing that runway.  It's briefed along with the rest of the takeoff and departure.It may be a climb straight ahead, or it may be a turn to a specific heading, or may involve a more complex series of directions for terrain or other reasons (political boundaries such as the 38th Parallel in South Korea, for example).
If the crew has a reason to return and does not have an emergency that limits climb performance or aircraft control, the crew will notify ATC and either request priority handling (eg, declare an emergency or or urgency situation ("mayday, mayday, mayday," or "pan, pan, pan"), and will be directed accordingly.  It may be an ill passenger, and the flight may follow the original routing and then be given a turn, or vector, to join the final approach course.
During that time, the crew will follow their normal procedures, if an emergency does not exist, including the relevant checklists and cockpit flows (steps the crew takes at each phase of flight, before doing the checklist).  If an emergency does exist, the crew will address the memory items necessary to handle that emergency and stabilize it, then the relevant abnormal or emergency checklists, then the normal checklists, and then arrange the return for landing, as appropriate.
Because many flights depart at a weight that is too heavy to land, the flight may need to be vectored to a point where fuel dumping may occur, to reduce weight to a landing weight.  In some cases, there may not be time, and the flight may have to accept an overweight landing.  In that case, the airplane will be grounded after it arrives, pending an inspection.
Properly done, in most cases, the flight will end up looking like a big rectangle, climbing out, with a crosswind, downwind, base, and final leg, to land.  Generally, a crew will set up the aircraft for a return to land, as part of their departure preparations, and will have briefed the return to land, and will know in advance their dump time and other particulars of the return.  This is all done before the aircraft has pushed back from the gate, or started engines.


text:
Are manned balloons allowed to land anywhere?
From time to time, quite spectacular videos turn up from hot air balloons landing in unusual or even quite tight spots, such as on a soccer field in Vlijmen, on a road in Nijkerk, or even on a small playground in Utrecht (all in The Netherlands).
While I appreciate the fact that only the height can be controlled directly and not much else, I can't help but wonder if it's safe to do so, and if it's allowed.
I've tried searching, but all that turned up was information on landing on a farmer's field. That is allowed and there are rules for handling that.
Are there any regulations concerning landing in built-up areas? Or can a balloon land anywhere? Or may a pilot be sanctioned afterwards for getting into a situation where they needed to land in such a spot?
I'm mainly interested in the Dutch law and regulations, but wouldn't mind learning about those in other countries. I'm also curious to learn what the considerations are when choosing a landing spot.

For the Albuquerque Balloon Fiesta, you get told not to land on the airport/Air Force base to the south. The security forces really don't like it.

The thing with unpowered aircraft (balloons, gliders, but also airplanes with a power emergency) is that the laws of physics don't care much about the laws of men. If you're going down, you're going down, and you sort out whether you were allowed to go down where you went down after you survived the landing. The third video is titled "emergency landing" if my one and a half words of Dutch aren't failing me, so it clearly was either land in the playground and live or crash into the buildings and (potentially) die.

From the few countries I looked at, the general rules are:

One can fly anywhere, as long as enforcing regulated and prohibited areas like other pilots.
Regulated areas include inhabited areas, there is at least a minimum distance and often a minimum height.
Pilots must plan their landing in authorized areas, taking elements into account. An emergency landing is still lawful, like for other types of flights. Many or most are due to pilot delaying the decision to terminate the flight.
Pilot incapacitation may lead to emergency landing (by another person than the pilot), or to crash landing. Recently FAA was pushed by NTSB to act, at least on medical conditions required to fly.
Pilots are supposed to be familiar with the related rules and NOTAMs.
When it can be proved pilots have been careless or reckless, they are sanctioned.

I'm focusing on heated air balloons and excluding tethered, unmanned and high altitude balloons

A balloon pilot must plan their landing according to weather forecast, on a safe area, and only on a private property for which they have the authorization to land. When the intended flight cannot be followed because of a failure, because being short of gas, or because of atmospheric conditions, then an emergency landing must be carried on. As balloons have no control surface, that means the landing area cannot be selected, except by accelerating the descent or slowing it as long as possible (heating air or jettisoning ballast).
After the facts, in case of damage, if any person was injured or killed, if any complaint is issued or just if police is informed of the landing, then it's a matter of proving the pilot acted carelessly or recklessly.
If this is the case the pilot incurs civil and criminal penalties. E.g. pilot not acting in time in this event New Zealand:

K... v Civil Aviation Authority involved a fatal
incident. K... was convicted of breaching a former
civil aviation regulation by flying a balloon in
circumstances whereby avoidable danger to life or
property was likely to ensue. K... was piloting the
balloon when it was blown 200 metres offshore. The
balloon landed in the sea and three people
died. The trial judge found K... had failed to keep
proper lookout for weather changes during  the
flight.  K...  was  acquitted  of  one charge  of
careless flying.

As for whether emergency landings are hazardous: They are, in particular the velocity and the vertical position of the gondola are not controlled. There are online accounts for serious injuries after uncontrolled landings and statistics from EASA (all EU countries, also includes collisions):
.
EASA statistics on balloon incidents and accidents, source
I was not able to determine exact rules for Netherlands, not even a summary of rules applicable to EU countries, however I'll give you the links to the legal framework for EU. But to start with, there are two approaches: Australia is rather liberal, no license is required, while US includes hot air balloons in general aviation aircraft.
Australia, regulated by CASA
In Australia commercial activities require a license but recreational ballooning is not regulated.

Balloon pilots can only land on private property if they, or their
support vehicle driver, have permission from the property owner.
Permission is not required in emergencies, or if no other landing site
is available.
Pilots must plan their flight based on forecast meteorological
conditions. However, changes in wind conditions could alter the
balloon's navigational tracking from that originally planned.
When over populated areas, balloons must fly at least 1000 feet above
ground level. When over other areas, balloons do not have to fly at a
minimum height

Source: CASA.
US: Regulated by FAA

Perfectly lawful emergency landing in Illinois, source
Hot air balloon operations are conducted in accordance with FAR Part 91 (general aviation) and pilots must hold a license according to FAR Part 61, with a minimum of 10 hours of flight, 35 hours for a commercial pilot (more).
FAA issued a AC summarizing balloons operations.
Ballooning operations are regulated like other GA operations. In particular access to airspace of different classes follows the same constraints. Most flights occurs in class G airspace (uncontrolled) or E (controlled) and below 14,500 ft and all under VFR. FAR 91:119 still specifies minimum safe altitudes:

(a) Anywhere. An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.

(b) Over congested areas. Over any congested area of a city, town, or settlement, or over any open air assembly of persons, an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft.

(c) Over other than congested areas. An altitude of 500 feet above the surface, except over open water or sparsely populated areas. In those cases, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure.

The first paragraph requires the pilot to be able to execute an emergency landing at any time to prevent an accident.
Netherlands: Mostly regulated by EASA, with local details
As said, I was not able to find a summary of rules applicable to balloons in Netherlands, so you may need to read the highly indigestible documentation from EU, the main problem with the text is there are constant abstruse references to other texts, and seemingly no single composite version.
The base for aeronautical operations are the Standardised European Rules of the Air (SERA), complemented by the recent EC 2018/395. A license is required (EC 1178/2011). It includes a knowledge test in different domains, a medical certificate and 16 hours of flight, 50 for commercial flights (more).
Then there is a local act on aviation, The Wet Luchtvaart.
About flying areas, it seems the principle is similar to US, balloons are considered aircraft. Maybe specific to Netherlands, locations used by balloons for launch and landing are subject to authorization from the municipality.

usa streets and parks and farms and whatever are the norm for hot air balloons.   Understand this is a sport balloon not a utility aircraft for going from point a to point b.  Which means you generally have one or a few launch spots that you always use, and you fly the same area that you have always flown, so you know where there are open spots with access, based on the ballooning community and the locals, there are places that are known to the community to be balloon friendly and places that are not.
We are trained, or should be, to recognize crops and where they are, to know if the field is freshly planted or not in use.  One should if possible land on the access roads and not on the crops, even if not in use, the crew should avoid driving across the field and even walking on the field.  The access roads are often next to trees or other obstacles.   If you do land on a field you should attempt to find the landowner, etc, etc.   Likewise with livestock, and how to fly or land near different types of livestock.
Usually, if not as a rule, you are trained by someone local and by the time you have hours or have purchased a balloon, you are somewhat familiar with the area, good launch spots, directions you should never fly for various reasons, etc.    When you go out of town it is generally for a rally, and organized rallies generally have faa waivers for height above ground, aircraft to aircraft, etc.  but also the organizers generally provide a map of the area and have red zones for balloon unfriendly properties.  these are locals that fly this area often and know it like we know our own areas.   and if you go to this rally year after year you begin to learn that area as well.
I have landed in back yards in desperation and a walmart parking lot once.  I have seen pilots land in soccer fields during a match.  not a good idea, but perhaps like no other aircraft, depending where you are (west mesa of albuquerque is generally super easy to fly, not only does it have great ballooning weather and the box, but out on the mesa nothing but open space) the moment you launch you are working the landing, some places are much harder to fly than others and you need not only the steerage skills (yes you can "steer" a balloon) but experience.
normally you look for open, unused land with access and no obstacles.  depending on where you are the edge of the city is great because it is likely government or unused land where the owners will not have a problem if they even bother.  some folks are very excited that you landed there vs angry.
I have not flown in other countries to know how it works, but if there is a local community then that community will know the local "rules" if you will, where the launch spots are where the landing spots are and most important which directions not to go and what land to avoid and by how much.
our "steerage" is based on the wind and flatlands it is often one direction so steerage means you put a pi ball up and draw a straight line from your possible launch spot and look on the map for landing spots.   if there is steerage either due to drainage or due to wind direction due to mountains, etc.  then you use that steerage as well as experience or maps to land in places you are familiar, if possible.

"Put a pi ball up" -- care to elaborate/clarify?  I'm guessing this might be a reference to launching a toy balloon to check the wind?

yes, not sure on the spelling.  releasing a latex balloon or two and watching them to see the wind layers to get a feel for the speed and steerage at the time.

yes, toy balloon, birthday party balloon.  helium filled latex balloon.  a number of websites spell it as pie ball


text:
Are the emergency exits on a Boeing 747-400 plug doors?
Rossiya Airlines flight FV5625 to Antalya suffered an incident at the gate when a passenger opened an emergency exit (seems to be the L3 door, from what I can tell) "because they felt too hot", deploying the slide.
In a discussion on the news item, someone remarked that it was lucky that it didn't happen in flight, which of course should be impossible.
But that made me wonder whether that emergency exit is a plug door, or if there's another mechanism preventing the door being opened in flight.

What is curious is the door slide was armed at the gate (it was not "on the runway").

@mins, you're right, I'll correct that.

@mins It is not unusual to have the doors armed at the gate. At least for some airlines it is procedure to arm all doors before the engines are started to make sure the aircraft can be evacuated quickly in case of a fire during starting.

@Bianfable: Makes a lot of sense of course.

I always thought that armed was the default state of all doors. "Armed, unless..."

Yes, all 10 passenger entry doors on the 747-400 main deck are plug-type doors:

The main deck passenger entry doors are used to enter and exit the airplane, and
serve as emergency exits. The ten passenger entry doors are paired along the
airplane fuselage. [...]
The entry doors are translating, plug-type doors. During opening, the door first
moves inward and upward, then translates outward and forward.

(Boeing 747-400 FCOMv2 1.50.3 - Airplane General, Emergency Equipment, Doors, Windows - System Description, emphasis mine)
This will make accidental opening of these doors while the aircraft is pressurized impossible.
Based on the picture from the video it was the L2 door, right in front of the wing (L3 is the overwing exit):

(YouTube: Passenger deploys emergency slide on Rossiya Airlines 747)
The passenger who opened the door probably did not know about the ARM state of the door:

Passenger Entry Door 1, 2, 4, and 5 Slide/Raft Operation
When the door mode select lever is in ARM position and the door operating
handle is rotated 180 degrees, the door begins to open and the power assist
opening system activates.
The flight attendant must release the door operating handle and continue to assist
the door opening motion by using the handles on the door and on the door
surround panel until the door is in the full open and latched position. The
door-mounted escape slide/raft deploys and inflates. [...]
When the door is to be opened from the interior and slide deployment is not
desired, the door mode select lever must be in DISARM position.

(Boeing 747-400 FCOMv2 1.50.3 - Airplane General, Emergency Equipment, Doors, Windows - System Description)

Do you know what is the pressure differential required for preventing the door to open airborne?

@mins I guess that depends on how strong you are. If the door is about 1m² in area (not sure how large it exactly is), then you'd need about 7kN of force to overcome 1psi pressure difference, equivalent to lifting up about 700kg.

It's going to be 1m2 at least, probably closer to 1,5m2 based on this short clip of a 747-400 door. Apparently finding the exact dimensions of cargo doors is a lot easier than finding those of L1-L3 doors.


text:
Can an Iranian citizen attain ATP in the United States?
I know that you can be a pilot in the U.S. as a foreigner. However, are there any special regulations regarding citizens of Iran because of political issues?

Hey Keith. I wanna thank you for your comprehensive answer. It was very useful and to the point. Just to clarify I already reside in U.S as an international student and have attended a four year aviation, pro flight program. I'm currently working on my Instrument Flight Rating and feel extremely fortunate to make my dream come true. I was also wondering, if you know of any U.S based airline that is likely to hire me with a H1B visa.

The U.S. does not prohibit Iranian citizens entering the United States and remaining here on extended business. It also allows noncitizens to attend flight school, conditional on a review of the candidate by the Transportation Security Administration. So, the U.S. would not categorically prohibit an Iran citizen learning to fly and obtaining a commercial or airline pilot's license.
However, while not impossible, it will be much harder than I just made it sound if you are living in Iran right now, because communications between Iranians and the U.S. State Department have been made difficult by the politics of the past 35 years.
There is, to put it delicately, no functional United States Embassy on Iranian soil. The U.S. maintains a virtual Iranian Embassy online at http://iran.usembassy.gov/ (also in Farsi at http://persian.iran.usembassy.gov/); however it was blocked by Iranian information authorities less than 12 hours after it initially opened and remains blacklisted to this day. Users with a VPN tunnel outside Iran's firewall can access it, but the site is mostly educational; you cannot apply for a visa using the site alone. You'd have to be able to submit your paperwork to a functioning U.S. Embassy or directly to the State Department offices in Washington D.C., which will be a delicate process as the Revolutionary Guard will be monitoring both sides of such correspondence.
Assuming you can establish reliable contact with the U.S. State Department either directly or through an Embassy, you would need an extended visa. There are three basic reasons to stay longer than 90 days, with visa classes for each; work, school, and temporary asylum. If you are coming to the U.S. specifically to learn to fly, an M-1 visa should be what you apply for; it permits entry to the U.S. for "vocational or other non-academic training", including FAA Part 61 and Part 141 flight schools. The Iranian authorities also look most favorably on student visas compared to other types.
In addition to the visa, you would, like any other foreign applicant for flight training, be required to pass a TSA security clearance. This is ultimately discretionary; you cannot force the TSA to grant your clearance (ask any U.S. national; you can't force the TSA to do anything).
While neither the State Department, TSA nor the FAA would categorically prevent you learning to fly based on country of origin, the TSA does list Iran as a country of origin that prohibits the visa holder from "Automatic Revalidation". Essentially, if while attending flight school, you ever left the U.S. (to visit family back home, or even driving north to Canada or south to Mexico for sightseeing) you would be required to go through the full security clearance process again including the TSA check. So, you would have to stay in the United States political borders at all times while in flight school, or you could risk being denied reentry or resumption of your flight training.
While taking flight training and amassing your 1000 hours flight time required for an Airline Transport Pilot license (that 1000 hours number is good only if you're attending a four-year flight school; for most civilians working their way up it's 1,500), there are a number of illegal acts you can unintentionally commit while flying. For citizens, the legal penalty for unintentionally breaking an FAA regulation ranges from nothing to a talking to by air traffic control up to formal hearings with possible suspension or revocation of your pilot certificate. For a non-citizen in the United States for the express purpose of learning to fly, any offense bad enough to get your pilot's license suspended would likely also get you deported (if not for the criminal act, then because you can no longer do what you're here to do). But, you'd usually have to mess up pretty bad several times to get that far.
Once you have your ATPL, your M-1 would no longer be valid as you'd no longer be a student, and so you'd have to leave the U.S. unless you made prior arrangements with the State Department and/or an airline to remain in the country under another status (such as H-1b to work for an airline). If you take your license and leave the country, the FAA ATPL would only allow you to fly "N-numbered" aircraft registered by the FAA. Most other countries have their own registration, and would require you to "convert" your pilot's license to the local regulatory agency's version before you could fly a locally-registered plane. For an ATPL holder this is typically easier than for lower levels, and for the EASA (the largest ICAO-compliant regulatory body besides the FAA) it boils down to a skills test (knowledge and practical), a new medical cert, and being re-rated by the EASA for multi-engine operations (the FAA AMEL class rating does not transfer).

I think this applies to those currently (or recently) resident in Iran.  There are a good many refugees living in the US or third countries who may still retain Iranian citizenship.

and another hurdle you've not mentioned is the requirement for a work permit. It's extremely hard for foreigners to get one. I'm not sure about the aviation sector, but in IT for example the total number of available H1b permits for a year tend to be handed out within hours, sometimes minutes, of the application process opening for a year due to automated systems in certain countries flooding the application system the moment it opens for business. Even if no such abuse happens in aviation, finding a US company to sponsor you as an Iranian would be hard.

M-1 is a type of student visa instead of a work visa, and these are somewhat easier to get because the applicants are not "stealing jobs from honest 'Muricans", they're here to "learn our ways and spread Freedom and Democracy through the world" (sarcasm mine). Flight school might be a tricky case to argue for, because an FAA flight school gets you an FAA flight certificate which by itself is useless outside U.S. NAS. You could argue that the FAA cert makes getting any other ICAO cert easier...

I suspect the OP, if he is himself a citizen of Iran, might already be living in the US or perhaps elsewhere in the world and is not (or not only) asking about visiting the US to undergo training but about actually working as a professional pilot in the US. The answer, while relevant and interesting, does not seem to cover this aspect of the question.

I don't think getting from Iran to the U.S. is quite as difficult as you make it sound. I've known several Iranians here (in the U.S.) who seem to be able to travel back and forth freely. I think all of the ones I've known were on student visas. Also, as a slight nitpick, the original question asked about an ATP, not just a CPL. An ATP requires a lot more than 250 hours (1,500 normally, 1,250 if you have an approved 2-year flight degree, 1,000 if you have an approved 4-year flight degree, or 750 if you previously flew for the U.S. military.)

Actually, the original question was recently edited; the original form of the question specifically stated "commercial pilot" so I answered in terms of a CPL. An ATPL for a foreigner would pretty much require that four-year program you mentioned, as you have to stay a full-time student for the M-1 to remain valid, so you can't take time off from flight school and rack up flight hours on your own.

However I will edit to answer the updated question.

The original question actually stated “commercial airline pilot” which suggests the OP was speaking in general terms rather than using proper terminology and was the reason for my interpretation. But then the OP did not come back to comment and the question was edited by someone else so what he actually meant is still somewhat unclear.


text:
What is "settling with power" and how does it differ from vortex ring state?
I've always heard these terms used interchangeably. I figured that "settling with power" was what the pilot did to cause the vortex ring state (VRS). But comments on another question indicate that they are two entirely separate aerodynamic phenomena.
I understand VRS, so what is settling with power?

Wikipedia seems to indicate they are synonyms.

@FredLarson The comments on this question say that's not correct

Maybe Simon needs to fix the Wikipedia page, then.

Seems to be some interested discussion here: http://helicopterforum.verticalreference.com/topic/10982-vortex-ring-state-vs-settling-with-power/

@FredLarson I've fixed a few wiki pages with correct info I've learned here on SE

I knew that the debate would start :)  This is probably the most contentious question in helicopter operations.  it's right up there with the "equal transit time" theory of lift generation a few years back.  The only agency I am aware of that defines them as different is Transport Canada.  Instructors, pilots, WikiPedia, training videos, books and blogs all get this wrong often. It seems to be very common in the US for people to refer to vortex ring state as "settling with power".
Tom, I know you said you already know what vortex ring state but I think that the "why are they different" explanation is better when both are contrasted together. Please excuse me for repetition.
They are different.
Vortex ring state (VRS).
VRS ring state is a purely aerodynamic condition. In normal flight, whether climbing, descending or cruising, the relative airflow striking the blades is principally from ahead.  In a developed VRS, the helicopter descends into it's own vortex which is "recirculated" as the helicopter descends faster than the vortex descends and results in the recirculated air being drawn up, then almost vertically down into the rotor.

From HelicopterEMS.com
This causes a critical increase in the angle of attack and a corresponding loss of lift causing the helicopter to descend faster and faster up to rates of 6000 feet per minute.
In order for the helicopter to descend into its own vortex, it must meet three conditions at the same time (typical values for most helicopters):

less than 30 knots airspeed;
  greater than 300 feet per minute descent;
  power applied to the rotor.

Removing anyone of these gets you out of VRS but there's a catch.  If you try to reduce the rate of descent by increasing collective and therefore power, which would be the instinctive reaction of someone not trained to recognise and get out of the state, you increase the strength of the vortex and the recirculation and make the problem worse.
The only way to stop the rate of descent in VRS is to use the ground, which you really don't want to do at up to 6000 feet per minute and will probably spoil your plans for the weekend.  However, you DO have power.  You can use all of it until you hit the limits and still you won't stop that descent.
To recover, remove power by entering auto-rotation or increase airspeed to fly out of the vortex.  Auto-rotating out of VRS can take a long time and use a lot of height in the recovery so the accepted and taught technique is to fly out of it, any direction will do, which takes a second or two to recover.
Settling With Power (SWP)
SWP is not an aerodynamic effect.
At it's simplest, it occurs when you need x amount of power to stop a descent but, for a lot of variable reasons, you have less than x power available.  Increasing power results either in exceeding the engine or power train limits or stalling the rotor as drag increase exponentially and exceeds the power available to drive the rotor.
The slower a helicopter flies, the more power it requires:

From the AOPA blog.
When you get into the low speed regime, you can see that the power required rises rapidly.
Imagine you are coming into a hover.  You are travelling slowly at about 20 knots but have a low rate of descent, not enough to enter VRS.  If you keep going forward, you will be OK.  But, our imaginary pilot makes a mistake and turns into a 15 knot tailwind. The airspeed drops to about 5 knots so the lift produced drops by the square of the speed change and the power required rises rapidly.
The helicopter will very quickly develop a high rate of descent. The pilot increases collective to compensate but finds that the engine cannot produce enough power to maintain lift and power the tail rotor which is also working harder since you increase the torque by increasing power.  It is possible to simply run out of power.  If the pilot is lucky, they will be unconscious and found with the collective lever buried in their armpit.
SWP nearly always happens close to the ground and once in it, there is no recovery (insert suitably large number like 99% of the time).  You have no spare power to reduce the rate of descent or to increase airspeed to increase lift and fly away.
SWP is normally a result of pilot error (as above), mechanical problem leading to unexpected reduction in power available, wind shear causing unexpected reduction in airspeed or other environmental effects reducing lift available (coming into a hot and high hover for example).
Perhaps the easiest distinction to make is that you are already using all of your available power when you enter SWP. There is nothing left to give!

Does it look to you like SWP is what happened here? The winds shifted suddenly on him and it looks like he just did not have power to stop the descent. Doesn't seem like it was long enough to have entered VRS

@TomMcW Ouch!  Yes, it could be but I think impossible to know from the video. It could have been a downdraft coming down the mountain but, it might be argued that if that is so, then power required to maintain the hover, with the downdraft reducing the angle of attack, exceeded power available.   My guess is that it is SWP, precipitated by a pedal turn to the right, to face the mountain, which would have caused an increase in power required and possibly led to no more torque in the bank.

I've never read the ntsb report, but the tv commentators said that the aerodynamics of the mountain caused a shift from a strong headwind to a strong tailwind. I think the right turn was an attempt to get out of the tailwind. It's just got the overall look of a situation that was impossible to get out of no matter how skilled the pilot was.

@TomMcW Yep, that would do it and I agree, it looks impossible to get out of.

@TomMcW This article sounds definitive, quoting an interview with the captain and indeed, it was SWP.

The slower a helicopter flies, the more power it requires Correct me if I'm wrong, but that only applies to conventional helis with a tail rotor. Helis with coax, and helis with two tandem rotors, use the same power in cruise as they do hover, actually maybe a little more for cruise to overcome drag.

@DrZ214 I think you're wrong. The increased power requirement at a hover doesn't come from the tail rotor, it comes from the increased induced flow leading to a smaller aerodynamic angle of attack requiring a larger angle of incidence from the blades to produce the same amount of lift, causing an increase in drag (profile and induced), which requires more power to overcome.

An example of SWP exactly as you describe:
httpss://www.onderzoeksraad.nl/nl/page/18001/ongeval-nh90-helikopter-aruba  (report in Dutch)

@Simon Can you clarify the distinction between the definition of "settling with power" you provided and the definition KorvinStarmast provided for "power required exceeds power available" in his answer?  Is it the same thing?

Settling with Power, Power Settling, and Vortex Ring State
This answer is from the basis of being trained to fly helicopters by the US Navy, about 35 years ago, and spending a few decades as a helicopter pilot.
The usage has changed over the years.  The point @Simon made about people not using standard terms doesn't help.  In American circles, an original distinction between the two terms, even when both had the word "power" in them, is that each addressed a two different case of

"I am descending"
with the phrase difference indicating

"why I am descending"

usually applied to a case where I try to stop the descent and it doesn't stop.  (Clear as mud, I know ... try having your check ride grade influenced by knowing the difference :p )
Power Settling meant -- "I am out of power, and thus I am descending (and RPM is drooping)"  That term is now taught as power required exceeds power available.
Settling With Power meant -- "I have more power available, but I am still descending even if I add that power."   This is the case of Vortex Ring State, and that is the term now used and taught in the Navy  (reference is the same link as above).
As @Simon points out, the gotcha in VRS that makes it lethal is that adding  power makes it worse.  The corrective action is to reduce collective pitch and fly out of it.
When the Navy taught me how to fly helicopters (81/82) our text book term "power settling" for the situation where we ran out of power and our rotor RPM began to droop, and "Settling with Power" to describe the effects of Vortex Ring state.

Vortex Ring State

This was faithfully and well described by @Simon.  Between the time I earned my wings and ten years later, when I was instructing in Seahawks, the Navy changed to use the far more accurate usage: VRS.  It was interesting to see the transition and how people got used to the new term.  The manuals I just checked out for V-22 and MH-60R also use that term.
So what is Settling with Power?
It's a confusing term, at best.
The continued use of "settling with power" is likely to do with where one learned how to fly, what country you are in, who your teacher was, and for how long someone used a term and may not want to change terminology.
Settling with power as a substitute for "power required excedes power available" is not universally accepted, although it appears to be a common usage in Canada.  To support that point, I offer you:
Exhibit A
Exhibit B

Unfortunately, the ad hoc definition of "settling with power" as a
purely power phenomena is not correct, in Canada or elsewhere. The
idea that such a definition is needed for "an approach where
insufficient power is available" is right on, but settling with power
is not the term.
Check here: http://www.dynamicflight.com/aerodyn...ettling_power/
{snip}
Evidence of the confusion caused by misuse of the terms is shown by
the confusion about recirculation, which is the cause of VRS/Power
settling/settling with power. It is the recirculation of the downwash
that causes all the misery of VRS.
We should actually agree on a term for the falling through on an
approach that is caused by insufficient power, and stick to it. ~ Nick Lappos

As regards expert advice, Nick Lappos is a well regarded helicopter aviation professional with over 4 decades of experience in the industry, and knows flying, test flying, rotary wing design, and much else.  You can look him up.  I find his take on most helicopter flying issues well grounded.  (OK, lousy pun...).

Welcome to Aviation.SE!

Settling with power sounds like a legal term anyway. Vortex ring state just has a good sound. We'll have to come up with something better for PREPA though

@TomMcW I like PREPA even though it's an acronym.  Radar and scuba were once acronyms, and we now use them as words.  Socialize Prepa? :-)

I could get used to it

PREPAre to crash?

Welcome to Aviation.SE.  I hope that you'll make a great contribution to the helicopter community here. We've successfully moved the whole community from "helicopters fly just like fixed wings except when hovering" to a much better understanding of the differences. Back to the topic, I think we're into semantics. My understanding is inline with that on this page where the meanings were confused by the US Army wanting to be different to the US Navy. Bottom line is that VRS and power required exceeding power available are distinct phenomena.

@Simon yeah, and confusing the two can get people killed.

"Settling With Power" is the same thing as "Vortex Ring State."
In the U.S.--where the term Settling With Power appears to have originated--it means the main rotor is in the Vortex Ring State.  It describes what happens to the aircraft when it enters MR VRS: despite having plenty of engine power available, the helicopter makes an uncommanded descent, or "settles."  In fully developed MR VRS, the available engine power is typically useless to stop the descent.  (FAA-H-8083-21A, p11-9)
This distinguishes the emergency from a similar uncommanded descent that occurs when there is not enough engine power available to keep the rotors turning.  This state could correctly be called "Settling Due To Insufficient Power," but is instead known as Overpitching.  In this case, as well, there is a difference in terminology for the same phenomenon.  In the U.S., overpitching is known as Low Rotor RPM. (FAA-H-8083-21A, p11-15)
In both cases, European terminology describes causes, while the U.S. terminology describes symptoms...but they refer to the same phenomena.
Cause: (Main Rotor) VRS
Symptom: Settling With Power
Cause: Overpitching (of the rotor blades, i.e. raising too much collective with not enough engine power available)
Symptom: Low Rotor RPM
It seems there's one more point of confusion, however.  In Canada, the term Settling With Power has also come into use, but to mean something like hitting the brakes too late.  When a pilot begins to decelerate too late during their approach, reaches the limits of their engine power available given the conditions, and cannot bring the helicopter to a stop, i.e. zero groundspeed, without an overtorque, and the result is either a hard landing or a go-around.
The real problem is a lack of standardized terminology across the industry.